Key is to have very similar first and second serve. The only difference is on the first serve racket goes forward and through the ball for pace, and on second one you want to hit upward and brush the back of the ball for arch and spin.

Nope!
I've been observing tennis for about 30 years. We're talking here about learning a consistent, directional, second serve. NOT about telling a pro tennis player how to hit his second serve.
If you don't have a second serve yet, you're still not quite comfortable serving with a continental grip. The conti grip forces you to pronate to hit a flat first serve, and you naturally chop the ball if you don't pronate. Use this chop effect to slice a second serve, same swingspeed, but chop and add slice to the ball.
Once you can hit a sidespin second serve, you start throwing the ball up more over your head, so you add topspin, but that's only after you're confident with a FAST swingspeed.
If you start with a topspin serve, having never sliced, you will lose confidence in your second serve because you never ever swung fast on a second serve and relied on it's spin to slow down the ball and get it in. Right in the middle of your second serve sessions, it will break down totally, and in a match, it will totally break down for sure.
Confidence in the swing speed first, THEN add the topspin / kick component.

Y indeed.
Why is the sky blue?
Why do stars twinkle?
I'll try. It's something I've observed.
Shorter players tend not to hit 140 mph serves. Meaning, their swingspeed is less fast than taller guys. With a fast swing speed, you want to be able to use that fast swing speed to get the ball IN, every time, on second serves.
Back to shorter players. Slower swing speed, can we agree? Function of less leverage and that sorta stuff...no disrespect for OliverRochus.
So a sidespin second serve can arc IN with assist from gravity, lack of ball speed, air resistance, BUT, a lower strikepoint leading to a smaller target area...which is the counter, but maybe here offset by the slower swing speed and slower ball speed.... MAYBE. We are speculating here.
And, almost as important, since shorter players generally tend to stay back and play a more defensive type of tennis, they don't NEED a really fast moving ball. In fact, a slower moving serve allows them to get in better defensive position to defend against a hard return of their serve!
Offense and defense require diferent criteria to succeed. Speed, surprise, quickness is advantageous to offense. Control is vital in defense. Control is helped when your serve allows you to get in defensive position before the return bounces into a corner of your court from the return.
Possibly, is all I'm saying....

At 13 with 8-months of experience, I would think it is unlikely that you are hitting a proper flat first serve. Your best bet is to develop a slice serve first. This can be used both as a first and second serve. It is easier to learn than a flat serve and more reliable. You will also be surprised how effective the slight amount of movement is in keeping your opponents from going on the offensive.

It will also force you to be in the "palm down" position at the top. This will make it much easier to transition to an effective flat serve with proper form. A topspin serve is very similar to a slice so this will be an easy transition as well.

Y indeed.
Why is the sky blue?
Why do stars twinkle?
I'll try. It's something I've observed.
Shorter players tend not to hit 140 mph serves. Meaning, their swingspeed is less fast than taller guys. With a fast swing speed, you want to be able to use that fast swing speed to get the ball IN, every time, on second serves.
Back to shorter players. Slower swing speed, can we agree? Function of less leverage and that sorta stuff...no disrespect for OliverRochus.
So a sidespin second serve can arc IN with assist from gravity, lack of ball speed, air resistance, BUT, a lower strikepoint leading to a smaller target area...which is the counter, but maybe here offset by the slower swing speed and slower ball speed.... MAYBE. We are speculating here.
And, almost as important, since shorter players generally tend to stay back and play a more defensive type of tennis, they don't NEED a really fast moving ball. In fact, a slower moving serve allows them to get in better defensive position to defend against a hard return of their serve!
Offense and defense require diferent criteria to succeed. Speed, surprise, quickness is advantageous to offense. Control is vital in defense. Control is helped when your serve allows you to get in defensive position before the return bounces into a corner of your court from the return.
Possibly, is all I'm saying....

Click to expand...

the sky thing and the star thing both have very straight forward explanations.

but the entire paragraph above sounds like total fallacy... you can basically switch the words 'tall' and 'short' around and it will still make the same amount of sense (or the lack of).

You're prolly correct....
BUT, given 30 years of watching tennis, and that mostly includes player's just starting out, NOT Laver, Barrasetchi, or Chang, there is a definete noticeable trend here.
Shorter player's DON'T hit as hard serves as tall players, and don't need to.
They win with defense and consistency, NOT with huge service winners.
Maybe you should do some positive thinking on your own, instead of just negatively reacting to everything your read.
And try some practical experience applications. Go to a tennis court, look at beginner's learning a conti grip serve, and what do you see?
You obviously never looked.

Im a junior player 13 years old been playing for about 8 months i rate myself about 3.5 and was trying to really develop a 2nd serve whats better topspin or slice serve? what easier?

Click to expand...

I'm only a 3.0, but a long time observer of tennis. Personally, I think topspin (ie., kick) second serve is better. But I'm physically not able to do that effectively at this time. Slice serve is, imo, easier, at least for me. and can be very effective at any level, imo. (think:McEnroe) Depends on the situation. Bottom line, learn and develop both. Here's a vid of a few of my second serves from recent 3.0 match play. As you might notice, my second serve motion and delivery is essentially the same as my first serve. I'm not skilled enough and don't know enough to recommend this, but at my level it's a weapon. Even against 3.5 players, who I generally still lose to.http://www.youtube.com/watch?v=v2SlPsVFVQE

I'm only a 3.0, but a long time observer of tennis. Personally, I think topspin (ie., kick) second serve is better. But I'm physically not able to do that effectively at this time. Slice serve is, imo, easier, at least for me. and can be very effective at any level, imo. (think:McEnroe) Depends on the situation. Bottom line, learn and develop both. Here's a vid of a few of my second serves from recent 3.0 match play. As you might notice, my second serve motion and delivery is essentially the same as my first serve. I'm not skilled enough and don't know enough to recommend this, but at my level it's a weapon. Even against 3.5 players, who I generally still lose to.http://www.youtube.com/watch?v=v2SlPsVFVQE

yes, isn't
boy he must be lacking on the other strokes, especially since some have said
the serve is so clearly the most important stroke!
Actually I'd tend to agree that he should never get broke at 3.0 with that serve, lol.